Web sensing mechanism for tracking systems

ABSTRACT

A web sensing mechanism for use with the edge of a moving web to sense its deviation from a desired predetermined path of movement. The mechanism includes a ring formed with a tapered inner edge engageable with the sensed edge of the web and which is arranged generally concentric with the axis of a roller around which the web is guided. The ring is mounted for rotation on the end of a control lever which actuates means for correcting the web deviation.

United States Patent Jordan 51 Oct. 17,1972

- [73] Assignee: Xerox [54] WEB SENSING MECHANISM FOR TRACKING SYSTEMS[72] Inventor: William E. Jordan, Penfield, N.Y.

' Corporation, Stamford, Conn.

22 Filed: Dec. 2, 1970 [21] Appl.No.: 102,489

52 vs. C! ..198/202 51 int. c|.- ..B65g 15/62 [58] Field of Search"226/23, 45; 198/202; 74/241- [56] 1 References Cited UNITED STATESPATENTS 4/1969 Wright et al. ..226/23 X $540,571 11/1970 Morse ..l98/2023,301,384 1/1967 Sowards ..l98/202 Primary Examiner-Allen N. KnowlesAttorney-James J. Ralabate, Norman E. Schrader and Bernard A. Chiama 57ABSTRACT A web sensing mechanism for use with the edge of a moving webto sense its deviation from a desired predetermined path of movement.The mechanism includes a ring formed with a tapered inner edgeengageable with the sensed edge of the web and which is arrangedgenerally concentric with the axis of a roller around which the web isguided. The ring ismounted for rotation on the end of a control leverwhich actuates means for correcting the web deviation.

4 Claims, 5 Drawing Figures PATENTEDnm 17 1912 SHEET 1 OF 5.

. INVENTOR WILLIAM 5 JORDAN ATTORNEY WEB SENSING MECHANISM FOR TRACKINGSYSTEMS This invention relates to sensing mechanisms and particularly toimprovements in a web tracking system for an endless belt. The trackingsystem arranged in accordance with the present invention is particularlyadapted for use with photoreceptor belts in automaticcopiers/reproducers that are constructed for high speed operation andrequire instant response to deviations of the belt.

In order to overcome many disadvantages for high speed copying, thelatest machine concept for copiers utilizes endless photoreceptor beltsmounted upon a plurality of rollers and wherein a series of images areproduced on the belt as the same is continuously moved about therollers. However, the use of photoreceptor belts require extreme care inmaintaining perfect alignment of the belt from processing station toprocessing station. Belt tracking systems utilized for this purpose mustbe near instantaneous in response to deviations and to utilize edgeengaging devices which will not damage the sensed edge.

It is, therefore, the principal object of this invention to improvecopiers/reproduction machines of the type having a photoreceptor belt byassuring accurate tracking of the belt without undue wearing of a sensededge of the belt.

Another object of this invention is to improve sensing mechanisms formoving webs by limiting the sensing function to one edge of the web andto minimize frictional wear on this edge.

These and other objects of this invention are obtained by means of acircular member mounted on an actuating lever which controls correctiveaction for moving a deviating web back to a desired predeterminedposition. The circular member includes a ring having a tapered inneredge engageable with one edge of the web and is adapted to follow theedge in either direction of deviation in order to effect correctiveaction in either direction. The plane of the ring is at an anglerelative to the plane of the edge of web in the vicinity of sensing sothat the ring is able to sense in either direction of deviation. Thering is mounted to one race of a bearing which has its other racesecured to the end of the actuating lever.

For a better understanding of the invention as well as other objects andfurther features thereof; reference is had to the following detaileddescription of the invention to be read in conjunction with theaccompanying drawings wherein:

FIG. 1 is a schematic sectional view of an electrostatic reproductionmachine showing the various processing stations;

FIG. 2 is a partial cross-sectional view of a belt assembly as seen fromthe front of the machine;

FIG. 3 is an isometric view of a portion of the belt assembly as seenfrom the front of the machine;

FIG. 4 is a partial sectional view of the belt assembly from the side ofthe machine with the belt mounted thereon partly in section to showvarious internal parts; and

FIG. 5 is a sectional view of a detail of the belt tracking system.

For a general understanding of the illustrated copier/reproductionmachine, in which the invention may be incorporated, reference is had toFIG. 1 in which the various system components for the machine areschematically illustrated. As in all electrostatic systems such as axerographic machine of the type illustrated, a light image of a documentto be reproduced is projected onto the sensitized surface of axerographic plate to form an electrostatic latent image thereon.Thereafter, the latent image is developed with an oppositely chargeddeveloping material to form a xerographic powder image, corresponding tothe latent image on the plate surface. The powder image is thenelectrostatically transferred to a support surface to which it may befused by a fusing device whereby the powder image is caused permanentlyto adhere to the support surface.

In the illustrated machine, an original to be copied is placed upon atransparent support platen P fixedly arranged in an illuminationassembly generally indicated by the reference numeral 10, arranged atthe left end of the machine. While upon the platen, an illuminationsystem flashes light rays upon the original thereby producing image rayscorresponding to the informational areas on the original. The image raysare projected by means of an optical system for exposing thephotosensitive surface of a xerographic plate in the form of a flexiblephotoconductive belt or web 12 arranged on a belt assembly generallyindicated by the reference numeral 14.

As will be described below, the photoconductive belt assembly 14 isslidably mounted upon two support shafts one of which is secured to theframe of the machine and is adapted to drive a belt 12 in the directionof the arrow at a constant rate. During this movement of the belt, thereflected light image of an original on the platen is flashed upon thexerographic surface of the belt to produce electrostatic latent imagesthereon at an exposure station A.

As the belt surface continues its movement, the electrostatic imagepasses through a developing station B in which there is positioned adeveloper assembly generally indicated by the reference numeral 15 andwhere the belt is'maintained in a flat condition. The developer assembly15 provides development of the electrostatic image by means of multiplemagnetic brushes as the same moves through the development zone B.

The developed electrostatic image is transported by the belt to atransfer station C whereat a sheet of copy paper is moved between atransfer roller in synchronism with the moving belt in order toaccomplish transfer of the developed image solely by the electrical biason the transfer roller. There is provided at this station a sheettransport mechanism generally indicated at 16 adapted to transportsheets of paper from a paper handling mechanism generally indicated bythe reference numeral 17 to the developed image on the belt at thestation B.

After the sheet is stripped from the belt 12, it is conveyed into afuser assembly generally indicated by the reference numeral 18 whereinthe developed and transferred xerographic powder image on the sheetmaterial is permanently affixed thereto. After fusing, the finished copyis discharged from the apparatus at a suitable point for collectionexternally of the apparatus.

Drive means is arranged to drive the selenium belt 12 in conjunctionwith timed flash exposure of an original to be copied, to effect imagedevelopment, to separate and feed sheets of paper and to transport thesame across the transfer station C and to convey the sheet of paperthrough the fuser assembly in timed sequence to produce copies of theoriginal.

The belt 12 comprises a photoconductive layer of selenium which is thelight receiving surface and imaging medium for the apparatus, on aconductive backing. The belt is journaled for continuous movement uponthree rollers 20, 21 and 22 located with parallel axes at approximatelythe apex of a triangle. During exposure of the belt 12, the portionthereof being exposed is that part of the belt run between the roller 20and the lower roller 21. As shown in FIG. 4, the photoconductive beltassembly 14 is illustrated with the photoreceptor belt 12 partiallybroken away removed in order to illustrate the assembly mechanismslocated adjacent the belt.

The upper roller 22 is rotatably supported on a hollow shaft 23journaled in two side plates 24 and 25, each having the generalconfiguration of a triangle. The upper apex of the side plate 24 isformed with an opening for containing and supporting a bearing 26 whichrotatably supports one end of the shaft 23. At the other end, the shaft23 isjournaled in a bearing 27 supported at the upper apex for the sideplate 25 in the same manner. Positioned within and along the hollowshaft 23 and projecting through a machine support frame plate 28 thereis a drive shaft 30 formed with a conical element 31 and a reduceddiameter extension 32. The drive shaft is supported on the frame by abearing 33 mounted in the frame and is connected to a drive mechanism(not shown). The conical element 31 is axially fixed and extendsoutwardly from the frame 28 so as to be mated with an inwardly taperedor chamfered end 34 of the hollow shaft 23 when the belt assembly ismounted on the frame. At the other end of the hollow shaft 23, the driveshaft 30 terminates in a threaded shank 35 which cooperates with alocking member 36 formed with a conical portion 37 adapted to mate withan inwardly chamfered surface 38 of the outer end of the hollow shaft23. The locking member is also formed with a rotary hand-operated knob39 for facilitating easy removal of the member for purposes to bedescribed below.

The purpose for the extended drive shaft 32 is to aid in supporting, aswill be described below, and for rotating the roller 22 and,consequently the belt, when the belt assembly 14 is in operatingposition; that is, when the side plate 25 is positioned against the mainframe plate 28. This arrangement also permits the easy removal of .thebelt assembly 14 from the machine frame.

The side plates 24 and 25 are maintained in parallel planes and rigidlysupported in spaced relation for supporting the rollers 20, 21 and 22and all of the other structures that comprise the belt assembly 14 byinternal structural plates 42 and also by the hollow shaft 23 by virtueof the ends of this shaft being fixed to the inner races of the bearings26, 27. The plates 24 and 25 are also spaced and held in position by ahollow shaft 43 which is identical to the rotatable hollow shaft 23except that its ends are secured to the side plates, and

does not rotate. At the end adjacent the frame plate 28, the shaft 43 ischamfered at 44 and arranged for cooperation with the conical portion 45of a support shaft 46 positioned axially in the hollow shaft 43. Theshaft 46 is secured to the frame 28 and has its outer end formed as athreaded portion 47 which cooperates with a locking member 48. Thismember is identical to the locking member 36 and is provided with aninwardly tapering or conical portion 49 which is cooperable with aninwardly chamfered surface 50 formed at the outer end of the hollowshaft 43. A knob 51 on the member 48 allows manual locking of the innershaft 46 relative to and within the hollow shaft 43. From the foregoing,it will be understood that the belt assembly 14 is mounted in cantileverfashion on the frame support plate 28 by means of the hollow shafts 23and 43, and that by virtue of the use of tapered or conical portions 31,37 in the case of the shaft 23 and the conical portions 45, 49 in thecase of the shaft 43, the belt assembly is adapted to be accuratelypositioned in its required location after movement of the assembly fromthe frame 28. It will be appreciated that this structure allows quickand easy removal and replacement of the belt assembly upon the framewhile still providing means for driving the belt and permit removal ofthe belt 12 from its supporting assembly.

The belt assembly 14 is provided with an arrangement for tracking thebelt 12 in the event it deviates laterally in either direction duringits movement around the rollers 20, 21, 22. The arrangement includes themounting of the roller 21 for rocking movement and an edge guidingdevice associated with the roller 20 for producing the rocking movementof the roller 21 in response to lateral deviation of the belt.

As shown in FIGS. 2 and 4, the roller 21 is secured to a shaft 55journaled in bearings secured to the ends of the parallel legs 56, 57 ofa yoke member 58. At the mid-point of the yoke member and extending inan opposite direction relative to the legs 56, 57, there is mounted ashaft 60 which serves to support the yoke member for limited rockingmovement about the axis of this shaft and to permit slight retraction.The shaft 60 is preferably in the form of a cylinder 61 of relativelylarge diameter and being rotatably mounted in spaced bearings 62 mountedon the frame 42 and having a coaxial reduced portion 63 secured to thelower cylinder 61 and secured within a suitable opening formed in thebight portion of the yoke member 58. A relatively heavy coil spring 64encircles the portion 63 between the yoke member and the lower surfaceof the lower bearing 62. Rocking movement of the yoke member 58 aboutthe axis of the cylinder shaft 60 will impart rotation of the cylinder61 within the bearings 62. The spring 64 imparts a continuous outwardforce upon the yoke and consequently the roller 21, when the belt 12 ismounted on its supporting rollers thereby placing the belt under slighttension during operation. The structural connection between the yokemember and the portion 63 is slightly loose in order to allow verylimited play between these parts to correct for slight edge to edgecircumferential variations in the belt 12.

Means are provided for retracting the roller 21 inwardly of the beltassembly to permit the easy removal and mounting of a belt upon theassembly. To this end,

the inner end of the cylinder 61 is connected to a quick-acting togglearrangement which will draw the cylinder inwardly so as to carry theroller 21 therewith to a retracted position. The.toggle arrangement alsoserves to retain the yoke member 58 and roller 21 in the belt assembly.

The upper part of the cylinder shaft 61 is formed with a tubularextension 71 axially aligned therewith. Suitable openings are formed inthe sleeve in diametrically opposed positions for supporting a pin 72which pivotally connects one end of a link 73 to this end of thecylinder 61. The link 73 is pivotally connected at its other end to oneend of another link 74 having its opposite end pivotally connected tothe frame structure 42. It will be apparent that the links 73, 74 andtheir respective connections form a toggle device which when actuatedinto alignment will cause the cylinder 61 to move outwardly and whenactuated into a buckled condition will cause the cylinder to beretracted inwardly. These actuations of the toggle links are produced bya drive link 75 which is pivotally connected at one end to the pivotconnection between the links 73, 74 and extends transverse of the beltassembly, through the outer wall 24 therefor and ending in a pivotalhandle 76 (FIG. 3) for actuating the link 75 axially for causingactuation of the toggle 73, 74 in either of its two operatingconditions. In order to remove or to replace a belt 12, the handle 76 issuitably manipulated to cause buckling of the toggle links 73, 74, whichaction draws the cylinder 61 inwardly to retract the roller 21. Thisforms sufficient slack in the belt as to permit an operator to move abelt easily relative to the rollers 20, 21 and 22. After a belt isplaced on these rollers for machine operation, the handle 76 ismanipulated in the opposite direction in order to place the links 73, 74in their aligned condition, as shown in FIG. 4, for forcing the roller21 against the belt 12 and locking the roller in this position. In thisoperation, the spring 64 serves as an overdrive and, as previouslymentioned, as a tensioning means for the belt. There is anotheroverdrive function for this spring, as will be described below.

As shown in FIGS. 2 and 4, the axis of the shaft 60 (cylinder 61 and theportion 63) is perpendicular to the axis of the roller 21 and in thesame plane therewith. This plane, also substantially bisects the anglebetween the planes of the belt runs between the roller 21 and each ofthe rollers 20 and 22. In the event the belt 12 starts to skew, in otherwords, to deviate from its predetermined set position with respect toall of its guide rollers, the resultant lateral movement of the beltwill be sensed by a sensing arrangement to be described below, and thiswill result in positive action being utilized to skew or rock the roller21 about the axis of the shaft 60 for returning the belt back to itsoriginal predetermined working position. This control skew is a trackingaction and will effect an angular relationship of the roller 21 relativeto the direction of movement of the belt thereby causing the same tosteer or follow the roller surface and be displaced laterally in orderto return the belt back to a centered position rather'than exertingpressure on the belt adjacent one edge portion thereof. In this manner,pressure is applied equally to all portions of the belt affected duringtracking action thereby minimizing the tendency of the trackingarrangement to adversely affect belt structure by exerting unduepressures of the belt structure adjacent one edge or the portion betweenthe mid-line of the belt and one edge. With the axis of pivoting of theroller 21 lying on the bisecting plane for the planes of thephotoreceptor belt runs, the deflection of the ends of the roller occursin opposite direction to provide maximum belt correction with minimumroller skewing. Preferably the roller 21 is covered with a rubbercoating which will prevent slippage of the belt as it steers duringtracking. During rocking of the shaft 55, both edges of the belt areaffected equally and, as the skewing increases during tracking action,any tendency of the belt to lessen in circumference will cause movementof the roller 21 inwardly against the tension of the spring 63 whichserves as an overdrive or a shock absorber for tracking action.

In the event that the belt 12 is removed and a new one applied to thebelt assembly which has a slightly larger or smaller circumference, thespring 63 will always maintain the same pressure of the roller 21 uponthe belt thereby insuring the same tension upon a belt regardless of itscircumferential size. The arrangement also eliminates any twodirectional forces being applied to the belt which could have adestructive effect upon the relatively thin film of the photoconductivematerial on the belt. In addition, with the axis of the roller 21, lyingon the bisecting plane of the angle between the adjacent belt runsduring rocking movement of this roller, there is a minimum ofdeflection, caused by skewing of the belt, along the exposure belt runbetween the rollers 20, 21 thereby minimizing the effect of belt skewingupon the imaging abilities on this run during an exposure of anoriginal.

Sensing means are provided in the belt assembly 14 in order to initiatecontrolled instantaneous rocking of the shaft 55 for the roller 21 ineither direction depending upon the direction of slipping of the beltaxially relative to the shafts of the rollers 20, 21, 22. Deviation ofthe belt is initially detected by a flat ring encircling one end of theshaft 81 which supports the roller 20 for rotation in the belt assembly.The shaft 81 is preferably hollow and is rotatably mounted at each endin bearings 82 mounted in the end walls 24, 25. The ring 80 ispreferably made of wear-resistant plastic material which will not becomedamaged when in contact with the relatively sharp edge of the belt 12,and yet will not cause fraying or other damage to the coacting edge ofthe belt. The ring 80 is mounted for rotation, which occurs duringcontact with the belt, in order to minimize wear of the ring. Rotationis accomplished by means of a ball bearing device comprising an innerrace 83 (FIG. 5) to which the ring 80 is axially aligned and secured andan outer race 84 which is secured to a circular plate 85 (FIG. 3) on asensing support structure for the ring 80. For ease of use, the plate 85is formed with a central opening 86 which allows the assembled structurecomprising the ring 80, the races 83, 84 and the plate 85 to accommodateand avoid the adjacent end of the shaft 81 and allow the ring 80 toengage the edge of the belt 12.

The support plate 85 is secured to curved arm 87 which in turn isintegral with the upper end of an angularly inclined shaft 88 mountedfor rotation along its longitudinal axis within a fixed cylindricalcasting 89. Spaced brackets integral with the casting 89 are mounted byscrews to the frame structure 42 for the belt assembly. Suitablebearings (not shown) are utilized within the casting between the sameand the shaft 88 in order to facilitate the rotating relationshipbetween these elements. At the lower end of the shaft 88, there isformed a reduced portion 91 which is rotatable with the shaft and towhich is attached an element 92 extending perpendicular relative to theaxis of the shaft so to swing in either direction upon correspondingrotative movement of the shaft. The element 92 is pivotally connected toa link member 93 which in turn is connected to an ear 94 formed at oneend of the yoke member 58. It will be apparent from the foregoingdescription that the roller 21 is adapted for rocking movement in atleast one direction when the sensing ring 80 is moved outwardly, or inthat direction away from the adjacent end of the roller 20. Upon thismotion of the sensing ring, the shaft 88 is rotated about its axis dueto its connection to the ring by way of the arm 87. The correspondingrotation of the shaft 88 will impart a swinging movement to the element92 to cause movement of the link 93, and consequently the rocking ofyoke member 58 for producing the same motion on the tracking roller 21.This final action on the roller, which is relatively slight, will effectmovement of the belt away from the ends of the rollers 20, 21 and 22which are adjacent the side of the belt assembly to which the sensingring 80 is positioned. In the event the ring 80 is moved toward theadjacent end of the roller 20, the roller 21 will be rocked in adirection that causes the belt to track toward the above referred toends of the rollers.

Movement of the sensing ring 80 is either direction in order to producerocking of the tracking roller 21 in either direction and thereby causetracking of the belt 12 in a direction which is opposite that in whichthe sensing ring moves occurs when the edge of the belt 12 deviates froma predetermined position relative to the guide rollers which support thebelt. For accurate alignment and positioning of electrostatic latentimages and then corresponding developed images with respect to theprocessing stations of the printing machine, it is necessary that thephotoreceptor belt maintain a constant and predictable path of movement.In the event the belt deviates from a prescribed path of movement, sayin the direction inwardly of the frame structure 28, the edge of thebelt will engage the surface of the sensing ring 80 and causecorresponding movement of the ring toward the frame structure, or towardthe right as viewed in FIG. 3. The amount and rate of this movement ofthe ring is consonant with the amount and rate of deviation of the belt.As described above, with the sensing ring being moved in this manner,the tracking roller 21 will be rocked in order to cause a countertracking of the belt so as to restore the belt to its originalpredetermined path of movement. Deviation of the belt in the otherdirection, that is, away from the frame 28, the sensing ring will followthe away moving edge of the belt in order to produce rocking of thetracking roller in a direction which will stop the deviation of the beltand to return it to its predetermined path. A light leaf spring 97having one end secured to the arm 87 and its other end maintained underslight tension against the frame 42 provides a slight force upon thering 80 in a direction toward the sensed edge of the belt in order toovercome the frictional forces in the sensing mechanisms and to insurethat the ring will follow the edge ifit deviates away from the ring.

The sensing arrangement and belt deviation control described aboveallows correction of belt deviation in either direction by positivelyactuating countermeasures in either direction, and to accomplish thiscontrol from only one edge of the belt. There is no need to utilize aconstant spring force or the equivalent which normally causes themovement of the belt in one direction by force by rocking a yoke memberin one rotation and to positively drive the belt in the oppositedirection so as to maintain the desired positioning of the belt. Thereis also no need for using two belt sensing mechanisms, one for each beltedge. The above described sensing and control arrangement provides adouble-acting single sensing control.

In addition, the sensing of belt deviation is accomplished at one rollerand the actual tracking is produced at another roller, the next rollerpositioned downstream of belt movement. In this manner, the time lag forcorrection of belt deviation is almost nil with only a portion of thebelt experiencing a deviation' There is no need for a fullcircumferential movement of the belt before correction is imposed, as isthe case wherein sensing and correction occurs at the same roller.

As previously stated, wearing of the relatively delicate sensed edge ofthe photoreceptor belt is held to a minimum by the use of a sensing ringthat is mounted for rotation along with and while in contact with theedge. In FIG. 5, the ring is shown to' have inner and outer diameterswhich provide a relatively large and long surface area for the edge toengage and to do so without slipping off the ring or to miss itentirely. Wearing of the sensed edge is also held to a minimum by virtueof the arrangement wherein edge sensing occurs at a roller which has itsaxis fixed, such as the roller 20, and whereat the belt is relativelystiff in a transverse direction so that it will not be able to buckle orcurl when it engages a sensing element. Sensing between rollers wherethe belt may be unsupported in a transverse line with a sensing elementin the path of lateral movement of the sensed edge as it deviates mayresult in a curling or buckling of the belt as it contacts the elementand before it has sufficient stiffness force to produce movement of thesensing element. This will result in a sensing activity that lackssensitivity, predictability and accuracy. In order to take advantage ofthe relative transverse stiffness of the photoreceptor belt on theroller 20, the sensing element takes the form of the flat ring 80 whichencircles the shaft for the roller at which sensing occurs and which hasa mean diameter approximately equal to the diameter of the arc of thebelt as it travels around the roller. It will also be noted that theplane of the sensing surface of the ring 80 is slightly canted therebyassuring that contact with the sensed edge of the belt always occurs atthe same place relative to the adjacent end of the roller 20 and thering 80. With the ring 80 being mounted on the inner race of a ballbearing assembly, the ring will rotate and along with the edge of thebelt when the same contacts the ring.

In order to prevent extreme lateral deviation of the belt during machineuse, there is provided a protection shut-off circuit (not shown) whichis connected to the main power source to the machine for shutting itdown with the occurrence of this sort of deviation and a switchmechanism 100 actuable in response to extreme belt deviation. The switchmechanism includes a switch arm 10] in engagement with the support plate85 for the sensing ring 80 and which serves to actuate a suitable doubleacting switch 102 in the mechanism 100. This mechanism along with thearm 101 is set to actuate the switch 102 to open the machine powercircuit in the event the arm 101 is deflected in either direction beyondpredetermined set limits, in response to movement of the sensing ring 80in either direction beyond prescribed limits. Such extreme deviation maybe caused by belt failure or sudden obstructions to its path of movementsuch as by a sheet of paper being carried around with the belt, orfailure in the structure of the belt assembly While the invention hasbeen described with reference to the structure disclosed, it is notconfined to the details set forth; but is intended to cover suchmodifications, or changes as may come within the scope of the followingclaims.

What is claimed is:

1. In a web tracking system for a moving web having a roller in contactwith the web to guide the same, and means for sensing deviation of anedge of the web from a predetermined position during movement thereof,

the improvement comprising an actuating mechanism for producingcorrective movement of the web in responsive to signals indicative ofweb deviation in either direction, said mechanism including a controllever pivotable in either direction and the pivoting of which in eitherdirection produces signals for said mechanism,

a circular member secured to the lever and adapted to engage the sensededge of the web for causing said pivoting of the lever,

said member including a ring element having a tapered edge engageablewith the sensed edge, said member including means for permittingrotation of the ring element as the same engages the moving sensed edge.

2. The system in claim 1 wherein the plane of the ring element is at anacute angle relative to the plane of the sensed edge adjacent said ringelement.

3. The system in claim 1 wherein said circular member also includes abearing device having two races one of which supports the ring elementand the other is mounted on said lever.

4. The system of claim 1 including means operatively associated withsaid circular member for stopping the movement of the web in the eventthe same deviates beyond a predetermined limit.

1. In a web tracking system for a moving web having a roller in contactwith the web to guide the same, and means for sensing deviation of anedge of the web from a predetermined position during movement thereof,the improvement comprising an actuating mechanism for producingcorrective movement of the web in responsive to signals indicative ofweb deviation in either direction, said mechanism including a controllever pivotable in either direction and the pivoting of which in eitherdirection produces signals for said mechanism, a circular member securedto the lever and adapted to engage the sensed edge of the web forcausing said pivoting of the lever, said member including a ring elementhaving a tapered edge engageable with the sensed edge, said memberincluding means for permitting rotation of the ring element as the sameengages the moving sensed edge.
 2. The system in claim 1 wherein theplane of the ring element is at an acute angle relative to the plane ofthe sensed edge adjacent said ring element.
 3. The system in claim 1wherein said circular member also includes a bearing device having tworaces one of which supports the ring element and the other is mounted onsaid lever.
 4. The system of claim 1 including means operativelyassociated with said circular member for stopping the movement of theweb in the event the same deviates beyond a predetermined limit.